Tremendous efforts have been devoted to epitaxially grow low defect density GaN layers since Nakamura and co-workers first reported the fabrication of GaN-based light emitting diodes (LEDs) in 1993 (S. Nakamura, M. Senoh, and T. Mukai, Appl. Phys. Lett. 62, 2390 (1993)). This is because of the difficulties in the growth of bulk GaN crystals. For example, a temperature of close to 2000° C. and nitrogen pressure of up to 20 kbar are needed to grow such bulk crystals. The limited crystal size further restrains the bulk technology for practical use. For the lack of adequate GaN substrates, epitaxial GaN layers are commonly grown on substrates to which they are both lattice and thermally mismatched.
The preparation of high quality active epitaxial GaN layers is the most important step for the fabrication of GaN-based LEDs. The most common and successful techniques to grow epitaxial GaN films (on foreign substrates) are metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE). Sapphire (Al2O3) is the most widely used substrate from the growth of hexagonal or wurtzite GaN. As-grown GaN layers contain a high density of threading dislocations because of the large mismatches both in the lattice parameters and the thermal expansion coefficients between the GaN film and the substrate.
Theoretically, cubic GaN possesses superior electronic properties for device applications since it provides high optical gain, (P. Das and D. K. Ferry, Solid-State Electron. 19, 851, (1976)) higher carrier mobility, easier p-type doping, and a narrower energy band gap. These properties make it easier to fabricate LEDs that can reach the blue and green regions. In addition, its cubic symmetry offers the isotropic properties that the hexagonal form cannot supply. However, such advantages have not been fully realized due to the difficulty in producing phase pure material. For instance, hexagonal GaN is thermodynamically stable. The cubic GaN can be only deposited on suitable substrates such as (100) GaAs, (100) MgO, (100) 3C—SiC, and (100) Si.
Even though the most commonly used substrate for the epitaxial growth of cubic GaN is GaAs, the direct nucleation of GaN on GaAs proves to be extremely difficult due to the short bond length in GaN compared to GaAs and the presence of native oxide on the surface of GaAs. The lower thermal stability of GaAs adds other limitations to using GaAs as a template for the epitaxial growth of cubic GaN.
Currently there are no practical routes to cubic GaN substrates and as such the advantages of LEDs prepared using this material cannot be realized.